Len is a Succubus Demon Familiar with the ability to manipulate dreams, created from infusing the soul of a deceased human girl into the body of a cat. As such, she is capable of shifting between the forms of a black cat and a prepubescent human girl. She once served a Magus several centuries ago, after whose death she was taken under the care of Arcueid Brunestud. Although she was under Arcueid's charge, because Arcueid is not human, they were unable to create a contract, therefore Len is not considered Arcueid's Familiar.

- Wikipedia

The translation of Len's name has been under the light of investigation for a long time, however the correct romanization of "Len" is reflected by Type-Moon's releases after the initial blunder in Kagetsu Tohya. The original translation of "Ren" was used incorrectly as there is no character for "Le" in the Japanese alphabet but has since been addressed in later releases featuring the character.

After events of Kagetsu Tohya visual novel her current contractor is Tohno (Nanaya) Shiki.